Zoroastrian tradition considers a dead body unclean.

To preclude the pollution of earth or fire, the bodies of the dead were placed atop a tower (a tower of silence) and so exposed to the sun and to scavenging birds.

A Tower of Silence (Dakhma or Dakhmeh) is a circular, raised structure used by Zoroastrians for exposure of the dead.

In the Iranian Zoroastrian tradition, the towers were built atop hills or low mountains in desert locations distant from population centers.
In the early twentieth century, the Iranian Zoroastrians gradually discontinued their use and began to favor burial or cremation.